The present invention relates generally to a system for monitoring the temperature within a kiln, and more particularly, to a system for measuring the temperature at a plurality of different locations on moving kiln cars.
Kilns have long been used in the manufacturing of ceramic articles. Just a few examples of the many articles (ware) that are made with the use of kilns are tiles, bricks, refractories, china, earthenware, electrical porcelain items, spark plugs, toilets, and bathroom sinks just to name a few. Products such as these are placed in a kiln to be baked or fired by temperatures far in excess of 1,000 degrees centigrade. Kilns in use today around the world come in various shapes and sizes to serve production needs of particular articles. For example, tunnel kilns can be in excess of 100 feet long having a track running through them for conveying multiple cars of articles to be fired in a mass production environment, while shuttle kilns have cars full of articles to be fired, moved into the kiln one at a time.
It is desirable to be able to monitor atmospheric conditions, such as temperature, carbon monoxide levels, oxygen levels, CO2 levels, humidity or water vapor levels, and sulphur oxide levels, inside of a kiln, at different locations or zones within the kiln, in order to optimize the firing process. High temperature thermocouples acting as temperature transducers may be used. Specifically, it is desirable to monitor the temperature in and around the articles or objects being fired.
In a tunnel kiln the objects being fired are in continuous motion though the kiln making it difficult to monitor the temperature in and around the moving objects. A plurality of cars carrying the articles to be fired, move on tracks through the kiln. Because the articles to be fired are resting on top of the cars, and since it is the object of the kiln firing process to fire the articles, it is not essential to heat the zone of the kiln in the vicinity under the cars. Therefore, this area is maintained at a much lower temperature than the firing regions above the car beds or decks. The underside of the cars are also somewhat insulated from the heat existing above the car beds due to the fact that the cars are often in end-to-end contact and heat seals may be placed on the sides of the deck of each car in close relation to the walls of the tunnel kiln. This can greatly reduce the underside temperature. However, the temperature under the cars can still reach a level up to 200 degrees Celsius.
It is known to use thermocouples to transmit temperature data to the exterior of a kiln. Some approaches have used either a data logger or a telemetry device to transmit the temperature data. A disadvantage of using a data logger is that the temperature data is not available until after the car exits the kiln. This precludes real time analysis and adjustment of the temperature within the kiln. Telemetry units also suffer from disadvantages. One is the fact that many tunnel kilns are made of steel which is not especially conducive to radio transmissions. Industrial radio telemetry links usually tend to be quite expensive. Additionally, because kilns are used in a variety of countries a radio-based telemetry link may face governmental regulation on the frequencies that could be emitted. A need exists for a real time temperature monitoring system for measuring, transmitting, and receiving temperature data from a variety of locations within a kiln.
The present invention may be applied to measure temperature and temperature distribution within a setting of articles being fired in a tunnel kiln or a shuttle kiln. The temperature data received will allow variation of operating processes by an operator of a kiln to reduce undesired gradients or otherwise to correct for temperatures which are either above or below the desired firing schedule. Two commonly used firing procedures are generally known as "continuous" and "periodic". Tunnel kilns and shuttle kilns, respectively, are associated with these firing procedures.
Tunnel kilns typically have a railed track much like a railway system. Cars which will carry the articles to be fired move on wheels over these tracks. A temperature profile is maintained in the space above the decks of the cars such that the entrance end and the exit end of the tunnel are at a much lower temperature than the center of the length of the tunnel. The cars generally move rather slowly through the kiln so that the articles atop each car experience a gradual rise in temperature to a maximum level which is subsequently lowered as the articles near the exit of the kiln.
Periodic firing was the more common method of kiln usage in the past. Under the periodic method the kiln is filled with ware, fired to its maximum temperature, cooled, and then unloaded. The drawback with this process is that major energy losses accompany it as compared to thermal efficiencies which are achieved under continuous firing. Several advantages do exist however, for periodic firing. These include the flexibility in production scheduling, variation in size and shape of the ware which may be accommodated, atmospheric and temperature control being more manageable, and improved thermal design efficiencies. More modern methods of periodic firing make use of shuttle cars to rapidly remove one or more cars from a kiln and place another car(s) to be fired into the kiln.
Thermocouples used to monitor the temperature at various locations within a kiln must be situated so as not to interfere with the passage of articles in the tunnel kiln, or with the insertion/removal of articles from a shuttle kiln. It would be advantageous to have temperature measurement throughout the setting of the articles being fired, such as at a point deep within a stack of ware, near the insulating deck of a car, close to the top of the setting of the ware between articles, and from side to side of the setting. The present invention is designed to accomplish such detection while the articles being fired go through the thermal process within the kiln.
Modern firing technology within a kiln is directed to the goal of optimum use of energy to achieve the desired fired properties for the articles. Detailed familiarity with the chemical and physical behavior of the ceramic material being processed is very beneficial to computing the optimum use of energy. Most ceramics undergo complex chemical reactions commensurate with their composition, thermal and gaseous exposure time. Heating often causes the decomposition or oxidation of mineral constituents with the evolution of gases and/or significant changes in physical size and bonding strength. Chemical reactions stimulated by intimate contact of the reactant particles at high temperatures generally create new compositions, some of which become liquid. The densification and strength development is usually accompanied by shrinkage and a reduction in the ability of the material to transport gases to and from the reaction sites within the ware. Temperature gradients imposed by the difficulties of heat penetration throughout the ware setting can result in variations in the fired properties of ware from different locations within the setting. Even worse, there may be bloated, cracked or warped ware as a result of such gradients, or strains which show up only in later service as spontaneous cracking.
The cooling phase can be disastrous to fired ware, since the material is now rigid and generally behaves in a brittle fashion. Thermal gradients accompany the cooling process, so there are thermal strains generated within the ware due to differential shrinkages. Some mineral constituents undergo rapid phase changes over narrow temperature regions, often associated with relatively large change of physical size. If such materials are subjected to temperature changes which are too rapid, they will generate major failure cracks during the cooling.
The above considerations lead to the design of firing and cooling curves, including "soak" times at constant temperature, which will result in the maximum production of first quality finished articles in the minimum time with the least amount of fuel for the firing process. To help attain these goals a detailed knowledge of temperature variations throughout the ware setting may be obtained so that processing techniques may be introduced to minimize such variations and their effects. Many modern burner systems utilize pulse firing and/or high velocity hot gases directed to the elimination of gradients. Variations in patterns of ware setting may be developed to alleviate some of these problems. Limits on the rate of heating are often established on the basis of unavoidable gradients. Whatever method is used, it is required that temperature measurements or other means of detection of thermal gradients be utilized to best accomplish these goals. With temperature data available during the firing process it is possible to directly observe influences of adjustment which are made. The present invention offers a system by which this result is achieved.
The present invention comprises non-contacting transmission of data from a remote location, such as under a kiln car, to a base station which may be located at any convenient site within a factory, via a magnetic coupling which may operate with negligible radiative output. The present invention comprises a plurality of thermocouples, signal conditioning electronics, a phase-change type thermal enclosure, a telemetry link using a nonradiating magnetic coupling, and a data display/recording device located external to the kiln.
The car mounted electronics package of the present invention includes thermocouple signal conditioning electronics. The resulting signals are converted into digital form and are read by a microcontroller. The microcontroller communicates with the base station via a special telemetry link. The base station is the other end of the telemetry link which may include a personal computer. Software on the base station computer may provide for the display and storage of the temperature data. The base station may be capable of communicating with many cars on a time-multiplexed basis.
The connection from the microcontroller to the base station computer may be an RS-232 format serial communications link preferably capable of operating at around 300 baud or greater. Each end of the telemetry link may contain both a transmitter and a receiver.
The transmitter and receiver are linked by an air core signal transformer of the present invention comprised of loops of wire which are arranged in such a way as to obtain good magnetic coupling between the base station and the mobile sensor unit(s). In a tunnel kiln application, the signal transformer winding may comprise several turns of wire wrapped around the perimeter of the car. The base station signal transformer winding (STW) may comprise a wire which is routed in a straight path through the tunnel kiln near one of the tracks, returning in a straight path near the other track. The transmitter may comprise a square wave oscillator which transmits when the RS 232 input signal is at logic one, and is idle when the input is at logic zero. The receiver may comprise a preamplifier and an AM demodulator circuit. A plurality of cars may use the same frequency under a software handshaking protocol, with the base station acting as the master. The telemetry link is that of an air-core transformer.
Other objects and advantages of the present invention will become more apparent upon consideration of the following detailed specification and drawings.